Method in manufacturing chemical or semi-chemical pulp from heavily resinous wood



United States Patent cc METHOD IN MANUFACTURING CHEMICAL 0RSEMI-CHEMICAL PULP FROM HEAVILY RES- INOUS WOOD Per Harald Collin,Upplands, Vasby, and Karl Esbjiirn Eftring and John Harald Jonsson,Skutskar, Sweden, assignors to Stora Kopparbergs Bergslags Aktiebolag,Skutskar, Sweden, a corporation of Sweden No Drawing. Application March21, 1955 Serial No. 495,752

Claims priority, application Sweden March 30, 1954 13 Claims. (Cl.16237) Since long ago it has been a desideratum in the celluloseindustry to make use of the heavily resinous kinds of wood of the Pinusfamily as a raw material in the production of chemical or semi-chemicalpulp according to the sulphite method. As is well known, it is possiblewith a normal sulphite method only to digest the sapwood but not theheartwood of the Pinus species. This depends on the contents ofpinosylvin of the heartwood, a substance which with the pH-valueprevailing at the digestion of a normal sulphiting batch condenses withthe lignin so as to make it impossible to dissolve the same out.

A number of methods for the digestion of Pinus species have beenpatented. As examples might be mentioned, inter alia, the British PatentNo. 5,365, the US. Patent No. 1,511,664 and the Swedish Patents Nos.77,429, 79,840 and 82,751. As far as we are aware, none of the methodsdescribed in these patents have found any technical use. This isbelieved to depend, inter alia, on the fact that with said methods thedigestion of the heartwood cannot be carried so far as the digestion ofthe sapwood, so that a pulp is obtained, which is not acceptable fromthe point of view of purity.

According to the methods described in said patents, the digestion takesplace in two steps, the wood being sulphited in the first step with anaqueous solution containing sulphite and/ or bisulphite ions and alkaliand/or alkaline earth metal ions, the temperature being augmentedcontinuously or stepwise to the desired level during the sulphitingperiod, whereupon the sulphited wood is hydrolysed with an acid agent,generally sulphurdioxide, in a second step.

If such a two-step method is used on a laboratory scale for thedigestion of Pinus wood and the sulphite treatment is effected at apH-value between 4 and 7, and if large quantities of the base are usedin the digester, such as 20-100 kilograms of Na O per kilogram of woodconceived of as being dry, pulps of relatively good properties can bemanufactured. However, it will then be necessary at the same time tomake use of very long digesting periods, generally 50-100% longer thanthose required in the digestion of ordinary spruce according to thecalcium bisulphite method.

Endeavours to apply the above described method on a technical scale,while the same time, with a view to reducing the base consumption,tapping of sulphiting liquor is effected after the impregnation, will inspite of all result in an irregularly digested pulp. The irregularity ofsuch a pulp clearly manifests itself, if the roe-number of the sapwoodand heartwood portions is examined. Differences in the roe-number of upto units will then be found. I

The method according to the present invention, however, constitutes acomplete solution of the problem of digesting heavily resinous kinds ofwood and has been successfully tested on a technical scale. The method,where the digestion also takes place in two steps, the wood beingsulphited in the first step upon steaming 2,885,317 Patented May 5, 1959a second step, is distinguished, furthermore, by the features that thesulphite treatment is elfected with the digester filled with liquorunder a hydraulic over-pressure during the greater portion of thesulphiting period, that sulphiting liquor adapted to be tapped is drawnoif entirely or approximately so and is used for the preparation offresh sulphiting liquid, and that the hydrolysis is brought about by anadapted quantity of sulphur dioxide being introduced continuously orstepwise into the digester.

According to the invention, a filled digester is thus operated withduring the sulphiting period, the liquor being held in the digesterunder a hydraulic over-pressure, resulting in that the total pressure inthe digester considerably exceeds the steam pressure of the liquor. Thefavourable influence of the hydraulic over-pressure is believed todepend, first, on the fact that air enclosed in.

the chips is pressed together by the pressure to a fraction of itsinitial volume so as to cause the digesting liquor to come into contactwith greater portions of the chips, and, second, on the fact that thesolubility of the air in the digesting liquor is multiplied, whereby alarge portion of the air enclosed in the chips can be released.

Experiments have shown that hydraulic over-pressures as high as possibleshould be used, that is to say, a pressure as high as allowed by thedigester is preferably applied, involving that 14-112 p.s.i. over thesteam pressure of the sulphiting liquor is made use of. Experiments haveshown, furthermore, that the hydraulic pressure should be maintainedduring the greater portion of the sulphiting period. Thus two digesterbatches on a technical scale, which were performed identically save forthe fact that the hydraulic over-pressure in one of the batches wasmaintained only one hour against 8 hours in the other batch, involved adifference of many hundred percent in the contents of splinters.

Experiments on a technical scale have shown that inert gases areaccumulated by degrees in the top portion of the digester during thesulphiting period, and it will be found suitable to release these gasesas they are formed, so that all chips in the digester are covered withliquor.

In addition to the abovementioned advantages, the hydraulicover-pressure results in that the temperature can be augmented veryrapidly during the initial phase of the digesting process. Thustemperature increases up to C. per hour may be permitted, without thepulp becoming more uneven thereby. The rapid temperature increase in theinitial phase of the digesting or cooking process brings about a shorterdigesting period and thus a better utilization of the digesting plant.

Technically, the hydraulic over-pressure may be arranged in such mannerthat the circulation conduit of the digesters is connected 'via a valveto a steam conduit, wherein a pump maintains an over-pressure. With theaid of a regulator adapted to release liquor from the steam conduit thepressure in the latter is maintained at the correct value. The liquorreleased by the regulator is preferably returned to the pump vessel. Theliquor under pressure may be constituted by sulphiting liquor fromearlier batches, by weak liquor from the washing of the pulp, or bywater. a

In digesting densely grown pinewood of a high age, for example, that isto say such pinewood as contains heartwood for the major portionthereof, it has been found suitable, with respect to the uniformity ofthe pulp, to supplement the pressure treatment by a preceding 'vacuumtreatment. Technically, such treatment may be carried into efiect asfollows: The digester is filled with chips with the use of a chipfilling apparatus operating with steam. When the digester is filled, thecover of the same isput on and the digester is connected to a suitablevacuum-producing apparatus such as a jet pump. Hereby the pressure inthe digester is lowered, until the wood water and the condensate formedin the chips during the steam process commence to boil. The steam thenformed entrains air enclosed in the chips, when it is sucked out of thedigester. Hereby the impregnation of the chips with sulphiting liquor isconsiderably facilitated. Furthermore, the vacuum treatment is believedto pull resin plugs out of the channels in the wood, which alsocontributes toward facilitating the impregnation.

It has been found to be suitable in connection with experiments on atechnical scale to effect the vacuum treatment in such manner that thepressure in the digester is lowered below 0.6 atmosphere, preferably to0.2-0.4 atmosphere, which corresponds to 60-75 C. At this latterpressure the temperature of, the digester contents has been lowered byapproximately -30 C. through the vacuum treatment. By the negativepressure prevailing in the digester after the vacuum treatment, theintake of sulphiting liquor is considerably facilitated.

As stated hereinbefore, large quantities of base should be used duringthe sulphiting period in the digesting of heavily resinous kinds of woodaccording to the method as outlined above. However, if a hydraulicoverpressure is used during the sulphiting period in accordance with thepresent invention, all sulphiting liquor adapted to be tapped may bedrawn otf upon the termination of the sulphiting period and be used forthe preparation of fresh digesting liquor. Hereby, it will be possiblein the method according to the invention to attain a low net baseconsumption in spite of the large quantities of base having to be usedduring the sulphiting period. By reason of the fact that only aninconsiderable portion of the wood is dissolved during the sulphitingperiod, all sulphiting liquor drawn oiI may be used for the preparationof fresh sulphiting liquor, without the risk of burnt cook beingincurred. Inasmuch as all sulphiting liquor permitting of being tappedis drawn olf upon the termination of the sulphiting period, very highdry substance percentages will be obtained in the liquor (up to 22%)after the hydrolysis. This is obviously a great advantage, inasmuch asthe costs for the evaporation of the liquor are considerably reducedthereby.

At experiments with the method on a technical scale according to thepresent invention, it has been found that the sulphite treatment willhave to be carried into eifect in different ways, depending on the kindof wood to be subjected to treatment, in order that pulps of the samestrength shall be obtained. If Swedish pinewood (Pinus sylvestris) isbeing used, for example, such wood having been stored in log piles for 1year, it has been found to be suitable to operate the sulphiting processat a maximum temperature of 135140 C., whereas fresh pinewood onlyrequires 130-135 C. and a sulphiting period shortened by 1 hour. Shouldthe more intense conditions be applied to the fresh wood, a pulp will beobtained, which is distinguished by a considerably lower strength thanthe pulp obtained with the stored wood under the same conditions.

To hydrolyse and to dissolve the lignin sulphited in the solid phase inconnection with the method according to the present invention,sulphur-dioxide is made use of. By reason of the fact that all orapproximately all sulphiting liquor permitting of being tapped is drawnoft" upon the termination of the sulphiting period, it is necessary toemploy an acid, which is gaseous at the hydrolysis temperature, in orderthat the acid shall permit of being uniformly spread over the contentsof the digester.

The sulphur-dioxide may be supplied to the digester in a gaseous orliquid state. In the latter case the sulphurdioxide is preferablysupplied in the circulation conduit,

whereby it is rapidly evaporated and distributed over the digester. Inthis latter case it will thus be preferable to keep so much liquor inthe digester that the circulation can be maintained.

In digesting Pinus sylvestris it has been found to be suitable tointroduce so much sulphur-dioxide into the digester that 34% of freesulphur-dioxide is obtained in the liquid phase. This corresponds at 130C. to a total digester pressure of 6-8 atmospheres overpressure. Ascould be expected, the hydrolysis proceeds more rapidly, the moresulphur-dioxide the digester is supplied with.

On account of the fact that the sulphiting process, especially inconnection with stored wood of Pinus sylvestris, will have to be carriedinto eifect at a relatively high temperature, it may be found suitable,with respect to the strength of the pulp, to lower the temperatureduring the hydrolysis period below the temperature prevailing during thesulphiting period. A certain temperature decrease is obtained already atthe tapping of the sulphiting liquor, since the latter is understood toboil oif and to form steam in the digester. If liquid sulphur-dioxide ismade use of, a further temperature decrease will be obtained through theheat absorbed by the sulphur-dioxide at the evaporation of the latter.In connection with experiments on a technical scale it has now beenfound that in spite of no heat being supplied to the digester from theoutside, the temperature is continuously augmented during the hydrolysisperiod. This raising of the temperature proceeds more rapidly in thebeginning of the hydrolysis period than is the case during the latterpart of the same.

If it is desired to maintain a constant temperature during thehydrolysis period, it will therefore be necessary to provide for coolingduring the same. One may also proceed so that the temperature of thedigester contents is lowered, prior to the introduction of thesulphur-dioxide, below the temperature at which the hydrolysis isintended to be carried into effect. By said temperature decrease, thetemperature level is then obtained during the hydrolysis period. Thesimplest method of effecting the lowering of the temperature consists inblowing out steam from the digester. The heat contents of this steam maybe utilized in various ways, for instance for preheating sulphitingliquor, as digesting steam, for the preparation of hot water, and soforth.

Pulp that has been manufactured in accordance with the method accordingto the invention must be relieved of resin by reason of its highpercentage of resin, before it can be used to advantage in the papermills. The removal of the resin may be elfected, for instance, bytreating pulp of a concentration of 1020% with approximately 10 kg. ofNaOH per ton of pulp in a defibrator, and by acidifying the pulp thathas been subjected to an alkali treatment, optionally after a portion ofthe liquor having been pressed off, to a pH-value of below 6 either bytreating the pulp with H at a high pulp concentration, or by mixing thesame with acidified water, so that the pH-value of the suspension isbelow 6. Hereby resin percentages below 1% in the finished pulp areobtained, while the production of resin spots in the pulp is alsoprevented.

What is claimed is:

l. A process for the manufacture of pulp from heavy resinous wood in adigestion zone comprising the steps of sulphiting said wood by steamingwith a liquor comprising an aqueous solution of at least one member ofthe group consisting of alkali and alkaline earth sulphites andbisulphites while maintaining a substantially constant hydraulicover-pressure of between 14 and 112 psi. over the steam pressure of saidsulphiting liquor throughout said sulphiting step and maintaining atemperature sufficient to augment said sulphiting, and thereafterhydrolyzing the resulting sulphited wood.

2. The process of claim 1 including the step. of lowering the pressureby creating a vacuum in a mixture of said wood and liquor, therebyfacilitating the intake by said wood of said liquor and thereaftersulphiting with said liquor while maintaining a hydraulic over-pressure.

3. The process of claim 2 wherein said pressure is lowered to below 0.6atmosphere.

4. The process of claim 2 wherein said pressure is lowered to between0.2 to 0.4 atmosphere.

5. The process of claim 1 wherein the step of sulphiting with saidliquor while maintaining a hydraulic overpressure is carried out duringa period of about one to eight hours.

6. The process of claim 1 wherein the step of hydrolyzing the resultingsulphited wood is carried out by continuously introducing sulphurdioxide into said digestion zone.

7. The process of claim 6 wherein the step of hydrolyzing the resultingsulphited wood by sulphur dioxide is carried out in the absence ofexternal heat.

8. The processof claim 6 wherein the step of hydrolyzing the resultingsulphited wood by sulphur dioxide is carried out at a temperature lessthan the sulphiting temperature.

9. The process of claim 1 including the steps of drawing off said liquorafter terminating said sulphiting and thereafter employing said liquorin the preparation of fresh liquor.

10. A process for the manufacture of pulp from heavy resinous wood in adigestion zone comprising the steps of sulphiting said wood by steamingwith a liquor comprising an aqueous solution of at least one member ofthe group consisting of alkali and alkaline earth sulphites andbisulphites while maintaining a constant hydraulic over-pressure of atleast 14 p.s.i. during a period of about 1 to 8 hours and maintaining atemperature sufficient to augment said sulphiting, and thereafterhydrolyzing the resulting sulphited wood with sulphur dioxide.

11. The process of claim 10 wherein said hydraulic over-pressure is 14to 112 p.s.i. over the steam pressure of said sulphiting liquor.

12. A process for the manufacture of pulp from heavy resinous wood in adigestion zone comprising the steps of sulphiting said wood by steamingwith a liquor comprising an aqueous solution of at least one member ofthe group consisting of alkali and alkaline earth sulphites andbisulphites while maintaining a substantially constant hydraulicover-pressure of at least 14 p.s.i. throughout said sulphiting step andmaintaining a temperature willcient to augment said sulphiting, loweringthe temperature of said digestion zone by blowing of steam therefrom andthereafter hydrolyzing the resulting sulphited wood by introducingsulphur dioxide into said digestion zone.

13. A process for the manufacture of pulp from heavy resinous wood in adigestion zone comprising the steps of sulphiting said wood by steamingwith a liquor comprising an aqueous solution of at least one member ofthe group consisting of alkali and alkaline earth sulphites andbisulphites while maintaining a constant hydraulic over-pressure of atleast 14 p.s.i. throughout said sulphiting step, and maintaining atemperature of from -l40 C., for a period of about 1 to 8 hours and andthereafter hydrolyzing the resulting sulphited wood with sulphurdioxide.

References Cited in the file of this patent UNITED STATES PATENTS 68,261Tiifany Aug. 27, 1867 480,334 Maste Aug. 9, 1892 1,761,544 Spencer June3, 1930 1,790,260 Cable Jan. 27, 1931 1,804,967 Wolf et a1 May 12, 19311,864,620 Richter June 28, 1932 1,903,962 Dreyfus Apr. 18, 19331,927,541 Bouvier et al Sept. 19, 1933 1,940,136 Haglund Dec. 19, 19331,986,760 Kreissler Ian. 1, 1935

1. A PROCESS FOR THE MANUFACTURE OF PULP FROM HEAVY RESINOUS WOOD IN ADIGESTION ZONE COMPRISING THE STEPS OF SULPHITING SAID WOOD BY STEAMINGWITH A LIQUOR COMPRISING AN AQUEOUS SOLUTION OF AT LEAST ONE MEMBER OFTHE GROUP CONSISTING OF ALKALI AND ALKALINE EARTH SULPHITES ANDBISULPHITES WHILE MAINTAINING A SUBSTANTIALLY CONSTANT HYDRAULICOVER-PRESSURE OF BETWEEN 14 AND 112 P,S,I. OVER THE STEAM PRESSURE OFSAID SULPHITING LIQUOR THROUGHOUT SAID SULPHITING STEP AND MAINTAINING ATEMPERATURE SUFFICIENT TO AUGMENT SAID SULPHITING, AND THEREAFTERHYDROLYZING THE RESULTING SULPHITED WOOD